The disturbing trend is also having an adverse effect on humans and animals, delaying the activities of marsupials and rodents and driving them into cover to hide from predators.

Some of the other effects include birds getting confused and flying into floodlit buildings, plants failing to grow properly and humans finding it hard to sleep.

About one-fifth of the world's population can no longer see the Milky Way, and it has already disappeared from Sydney skies, along with the Magellanic clouds.

The Southern Cross is also fading - already the fifth and faintest star is invisible from nearly everywhere in the city and the fourth is on its way out.

More than 90 per cent of the roughly 2400 stars that once would have sparkled above the CBD are now invisible, leaving only the brightest of them shining.

Amateur astronomers who want to enjoy the cosmos minus the bright lights of the city can join in next Sunday's Festival of Astronomy in North Sydney.

Local businesses will turn off their building lights so viewers can get a clearer view of the stars.

About 3500 attended the inaugural festival last year. This year more than 100 telescopes will be on hand at North Sydney Oval to allow festivalgoers to view the wonders of the night sky from 6pm to 9.30pm.

It's an ideal time to view Mars, with the "red planet" predicted to be at its closest point to Earth in more than 70,000 years in late August.

As part of the festival, Dr Crawford will give his free talk at the Darling Harbour Convention and Exhibition Centre at 1pm next Sunday, titled "Who has stolen the Milky Way?", which will offer some strategies for combating the consequences of light pollution.